The dolomite underlying gypsum-salt layer in the 5

th

member of Ordovician Majiagou Formation in the eastern Ordos Basin proves to be of great gas potential by latest exploration practice.Thus we conducted systematic study on its petrological features

reservoir space

porosity and permeability by core observation

thin section identification

physical property analysis and geochemical analysis.The results show that the reservoirs underlying the gypsum-salt layer in the Ordovician are of porous dolomite type in the Ordos Basin

and they occur vertically in O

1

m

6

5

O

1

m

7

5

and O

1

m

9

5

.The litho- logies of the O

1

m

6

5

reservoirs are mainly granular dolomite and porphyritic dolomite with very fine-crystalline

while the lithologies of O

7

1

m

5

and O

1

m

9

5

reservoirs are dominated by granular dolomite and dolomite with very fine-to fine-crystalline.The reservoir spaces for the three reservoir intervals are primarily dissolved pores or cavities

with residual intergranular pores

intercrystalline dissolved pores

intercrystalline pores and micro cracks ranked second.The average porosity for each is 5.84%

6.81% and 7.03%.Sedimentary paleo-geomorphology

sea level eustacy and penecontemporaneous meteoric water dissolution collectively control the development and distribution of the dolomite reservoir underlying gypsum-salt layer.It is concluded that the central paleo-uplift belt and Yulin-Hengshan uplift belt

located outside the boundary of the gypsum-salt layer

are relatively higher in topography

and the deposits of grain shoal facies are relatively thicker

so they are prone to undergo meteoric water dissolution during penecontemporaneous and shallow burial stages.Its internal pores are not easily subject to filling of gypsum salt.All in all

they are the favorable areas for the growth of dolomite reservoirs underlying the gypsum-salt layers inside the basin.